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The baryon asymmetry problem in physics refers to the apparent
fact that there is an imbalance in baryonic matter and
antibaryonic matter in the universe. Neither the standard model of
particle
physics, nor the theory of general relativity provide an
obvious explanation for why this should be so; and it is a natural
assumption that the universe be neutral with all conserved charges.[1] The Big
Bang should have produced equal amounts of matter and antimatter;
as such, there should have been total cancellation of both. In
other words, protons should have cancelled with antiprotons,
electrons with antielectrons, neutrons with antineutrons, and so on
for all elementary particles. This would have resulted in a sea of
photons in the universe with no matter. Since this is quite
evidently not the case, after the Big Bang, physical laws must have
acted differently for matter and antimatter.

There are competing theories to explain this aspect of the
phenomenon of baryogenesis, but there is no one
consensus theory to explain the phenomenon at this time.

Most explanations involve modifying the standard model of
particle
physics, to allow for some reactions (specifically involving
the weak nuclear force) to proceed more easily
than their opposite. This is called " violating CP symmetry" in weak
interactions. Such a violation could allow matter to be
produced more commonly than antimatter in conditions immediately
after the Big Bang.
However, as of yet, no theoretical consensus has been reached
regarding this, and there is no experimental evidence of an
imbalance in the creation rates of matter and antimatter.

Another possible explanation of the apparent baryon asymmetry is
that there are regions of the universe in which matter is dominant,
and other regions of the universe in which antimatter is dominant,
and these are widely separated. The problem therefore becomes a
matter/antimatter separation problem, rather than a creation
imbalance problem. Antimatter atoms would appear from a distance
indistinguishable from matter atoms, as both matter and antimatter
atoms would produce light (photons) in the same way. Only in the
border between a matter dominated region and an antimatter
dominated region would the antimatter's presence be detectable, as
only there would matter/antimatter annihilation (and the subsequent
production of gamma radiation) occur. How easy such a boundary
would be to detect would depend on its distance and what the
density of matter and antimatter is along it. Presumably such a
boundary would lie (almost by necessity) in deep intergalactic
space, and the density of matter in intergalactic space is
reasonably well established at about one atom per cubic metre.[2][3]
Assuming this is the typical density of both matter and antimatter
near a boundary, the gamma ray luminosity of the boundary
interaction zone is easily calculated. Approximately 30 years of
scientific research have placed boundaries on how far away, at a
minimum, any such boundary interaction zone would have to be, as no
such zones have been detected. Hence, it is now considered unlikely
that any region within the observable universe is antimatter
dominated.

At least one more major scientific study, called the Alpha Magnetic
Spectrometer, is planned that would, among other things,
advance our capability of detecting very distant antimatter
dominated regions,[4]
although it is facing funding problems.[4][5]

Another possibilty is that antimatter dominated regions exist
within the universe, but outside our observable universe. Inflationary cosmology models
suggest that the there may be more to the universe than can be seen
from the Earth, if only for the simple reason that the universe
isn't old enough for light from the most distant parts of the
universe to have reached us yet. If so, radiation from the boundary
of matter and antimatter dominated regions may simply still "be on
its way" to Earth, and so cannot be observed.